Molecular Basis of Inheritance

DNA is the genetic material in most organisms, and this chapter explains its structure, how it was proved to be the genetic material, and how its information is copied and expressed. You will study the Watson and Crick double helix with its antiparallel polynucleotide chains, Chargaff's rules, 10 base pairs per turn and a pitch of 3.4 nm, and the packaging of DNA around histone octamers into nucleosomes. You will follow the experiments of Griffith on Streptococcus pneumoniae transformation, Avery, MacLeod and McCarty, and Hershey and Chase using radioactive 32P and 35S, and learn why RNA was probably the first genetic material. The chapter then covers semiconservative replication proved by Meselson and Stahl using 15N, the replication fork with leading and lagging strands, transcription by RNA polymerase and processing of hnRNA by capping, tailing and splicing, the features of the genetic code, translation on ribosomes, and regulation of gene expression through the lac operon of E. coli. Finally, you will learn the goals and salient features of the Human Genome Project and the technique of DNA fingerprinting using VNTRs.

Detailed Worksheet: Molecular Basis of Inheritance Section A - Definitions (10 marks) 1. Distinguish between a nucleoside and a nucleotide with one example of each. (2 marks) 2. State Chargaff's rules for double-stranded DNA. (2 marks) 3. What is a nucleosome? How many base pairs of DNA does a typical nucleosome contain? (2 marks) 4. Define a transcription unit. Name its three regions. (2 marks) 5. What are VNTRs? Why are they useful in DNA fingerprinting? (2 marks) Section B - Calculations and Applications (15 marks) 6. A double-stranded DNA sample contains 20% adenine. Calculate the percentages of thymine, guanine and cytosine. If the molecule has 1,000 base pairs, calculate the number of hydrogen bonds between the two strands. (3 marks) 7. The haploid human genome contains about 3.3 x 10^9 bp. Calculate the length of DNA in a diploid human cell, given that the distance between two consecutive base pairs is 0.34 nm. Also calculate the length of the E. coli DNA of 4.6 x 10^6 bp. (3 marks) 8. In the Meselson and Stahl experiment, E. coli grown in 15N medium was transferred to 14N medium. Calculate the proportion of hybrid (15N/14N) and light (14N/14N) DNA after one generation, two generations and three generations. (3 marks) 9. The coding strand of a DNA segment is 5'-ATG TTC GGA TAC TGA-3'. Write the template strand, the mRNA transcribed, and the amino acid sequence using: AUG Met, UUC Phe, GGA Gly, UAC Tyr, UGA stop. How many amino acids will the polypeptide have? (3 marks) 10. The E. coli genome of 4.6 x 10^6 bp is replicated in about 18 minutes by two replication forks moving in opposite directions. Calculate the approximate number of base pairs polymerised per second by each fork (take 1 minute = 60 seconds). Name the enzyme that polymerises and the enzyme that joins Okazaki fragments. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of a replication fork showing the template strands, leading strand, lagging strand with Okazaki fragments, DNA polymerase and the direction of replication. (4 marks) 12. Draw a labelled diagram of the lac operon showing the i, p, o, z, y and a genes, and show the state of the operon in the absence of lactose. (3 marks) 13. Draw a labelled diagram of the secondary (clover leaf) structure of tRNA showing the anticodon loop, amino acid acceptor end, T loop and D loop. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Describe the Hershey and Chase experiment, explaining why 32P and 35S were used. Analyse the results to show that DNA, and not protein, enters the bacterial cell, and explain why the blending and centrifugation steps were essential. (5 marks) 15. In eukaryotes, hnRNA must be processed before translation. Explain capping, tailing and splicing, and analyse why the split-gene arrangement and splicing may represent an ancient feature of the genome. Compare this with transcription in bacteria. (5 marks) 16. Explain how lactose acts as an inducer of the lac operon. Predict what happens to the expression of the structural genes when (i) the repressor gene is mutated so that it cannot bind the operator and (ii) lactose is exhausted from the medium. Justify why the lac operon is said to be under negative regulation. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw all diagrams neatly in pencil and label every part. Always show 5' and 3' ends of nucleic acid strands.